Structural Vibration Analysis

Structural Vibration Analysis

Determining the Natural frequencies of the Composite beam having a patch of Piezo-electric layer using;

Transfer-Matrix Method, Rayleigh-Ritz Approximation method, &

ANSYS Modal analysis

1.5

1

300

40

70

20

Brass Piezo-patch

Steel Substrate layer of beam

Outline of Composite beam (not to scale). All dimensions are in [mm]

The beam is discretized into three parts - 1.) 7 cm from the left end is the first segment2.) 4 cm from the previous 7 cm segment is the second segment3.) 19 cm from the right end is the third one.

We have been presented with a beam which has clamped-pinned boundary conditions. We need to find first three natural frequencies of this composite beam.

• The beam material known as substrate has been made of Steel, while thePiezo-patch attached on the surface is composed of Brass. The following arethe engineering properties of the two material:

Young's Modulus [GPa] Density [kg/m3]

Brass 120 8600 Steel 200 7800

• Effective Properties for each segment;

Segment ρA [Kg/m] EI [Nm2] 1st 0.1560 0.3333 2nd 0.4140 4.1582 3rd 0.1560 0.3333

Method First frequency f1 [Hz] Second frequency f2 [Hz]

Third frequency f3 [Hz]

TMM 37.34 127.12 274.8 ANSYS Modal analysis 37.123 109.64 250.35

Rayleigh-Ritz Approximation

With different cases of Number of Admissible

shape function N included in the analysis

1 41.7796 - - 2 40.1182 233.6618 - 3 40.0296 173.7059 457.9501 4 38.8449 163.8290 456.4827 5 38.2748 145.2576 349.2241

10 37.7827 138.6602 299.3835 15 37.6458 136.0964 293.0809 20 37.5608 132.6028 284.7574 30 37.3985 130.2814 281.1560

- The following table lists down first three frequencies that are obtained fromthese three different methods.- For Rayleigh-Ritz approximation method, we are evaluating the values offrequencies for 9 different cases. This is done in order to study what number ofAdmissible shape functions would be required for the values of naturalfrequencies to converge close to the results from Transfer-matrix method.

- Purely base on extended run, it can be commented that the Number of shapefunction terms that must be included should be between 30 & 35.

First 4 mode shapes plotted through TMM code using MATLAB

Modal analysis done through ANSYS

1st mode shape at frequency of 37.123 Hz

2nd mode shape at frequency of 109.64 Hz

3rd mode shape at frequency of 250.35 Hz

4th mode shape at frequency of 441.53 Hz

Torsional mode shape at frequency of 462.07 Hz

5th mode shape at frequency of 659.58 Hz

The following set of graphs shows how the natural frequency values converges to the exact value from TMM when Rayleigh-Ritz analysis includes more number of Admissible Shape functions.

1.) For 1st Natural Frequency approximation

2.) For 2nd Natural Frequency approximation

3.) For 3rd Natural Frequency approximation

4.) Plot showing computation time -vs- Number of terms taken in the analysis

clc;clear all %% INPUT parameters% parameters listed are for "3 segmented beam" example from lecture

n = 3; %number of beam segments EI = [1/3,4.1582,1/3]; % effective EI for [segment 1, segment 2, segment 3] in N.m^2rhoA = [0.1560,0.4140,0.1560]; % effective rhoA for [segment 1, segment 2, segment 3] in kg/ml = [.07, 0.04, .19]; % length for [segment 1, segment 2, segment 3] in mBC = char('clamped-pinned'); %% Given Clamped - pinned Boundary condition

w_guess = [1310]; % Keep changing this value. 1st Natural Freq b/w 10 & 550, 2nd Nat. Freq b/w 550 & 1300, 3rd Nat. Freq beyond 1300.

%% Internal calculationsglobalx = [];for i = 1:n %sweeping through segments% Npaneli = [80, 45, 150]*1e3 + 1; Npaneli = l*1e3 + 1; r_s = linspace(0,l(i),Npaneli(i)).*1e0; %panels within a segment dr_s = (r_s(2) - r_s(1)); rmid_s = 0.5*dr_s + r_s(1:end - 1); rmid{i} = double(rmid_s'); x_vec = rmid_s; if i > 1

x_vec = rmid_s + sum(l(1:i - 1))*1e0; end% globalx = [globalx; x_vec']; %global r globalx{i} = x_vec'; %gend

%% Step 1. Calculating Field Transfer Matrix (FTM) for a Euler Bernoulli Beamsyms w xassume(w > 0)assume(x >= 0)FTM_matrix_size = 4;FTM_x_w = sym(NaN(FTM_matrix_size,FTM_matrix_size,n));FTM_w = FTM_x_w;

for seg = 1:n %sweeping through beam elements FTM_x_w(:,:,seg) = FTM_bending_class(w,x,EI(seg),rhoA(seg)); %

APPENDIX

Transfer Matrix Method code

FTM_w(:,:,seg) = subs(FTM_x_w(:,:,seg),x,l(seg));enddisp('FTMs evaluated')

%% Step 2. Calculating Global Transfer Matrix (GTM) for any system%GTM_w is a 2D Global Transfer matrix for the entire structure as a%function of 'w'

% [ PHI(L) ; [ PHI(0) ;% d(PHI(L))/dx; = [GTM_w]* d(PHI(0))/dx ;% M(L) ; M(0) ;% V(L)] V(0) ]

GTM_w = eye(FTM_matrix_size);for seg = 1:n GTM_w = FTM_w(:,:,seg)*GTM_w; enddisp('GTM evaluated')

%% Step 3. Applying B/C and finding Characteristic matrix

Char_Matrix_w = TMM_characteristic_equation_class(BC, GTM_w);Char_Matrix_w = vpa(Char_Matrix_w,5);% max_val = max(max(double(subs(Char_Matrix_w,w,1))));disp('[Bending]Characteristic matrix generated')

%% Step 4: Finding natural frequencies[w_h, nroots] = TMM_root_finding_class(w,Char_Matrix_w,w_guess);disp(['[Bending]Natural freq = ',num2str(0.5*w_h/pi,5),' [Hz]'])

%% Step 5: Finding Mode Shape

[PHI,PHIs,PHI_shape] = mode_shape_class(w_h,w,FTM_x_w,x,l,BC,rmid);plot_modeS = mode_shape_plot_panel_class(1,PHI,PHIs,n,'bending',globalx,BC,w_h);

function FTM = FTM_bending_class(w,x,EI,rhoA)%% CALCULATES THE INDIVIDUAL FIELD TRANSFER MATRIXES OF SECTIONS %Uses Cayley-Hamilton's theorey (reduces numerical error associated with expm command) %Elements of FTM from WickenHeiser Paper %Title: "Eigensolution of piezoelectric energy harvesters with

%geometric discontinuities: Analytical modeling and validation"%--------------------------------INPUT-------------------------------------% w - symbolic variable for circular natural frequency% x - symbolic variable for local beam length function% EI - scalar, effective bending rigidity value for segment% rhoA - scalar, effective mass/length value for segment%---------------------------------OUTPUT-----------------------------------% FTM - [4x4] OR [6x6] Field transfer matrix for the beam segment%--------------------------------------------------------------------------

Bet = (rhoA*w^2/EI)^0.25; %Beta for section

%% Mode Shape ConstantscoshBx = cosh(Bet*x);sinhBx = sinh(Bet*x);cosBx = cos(Bet*x);sinBx = sin(Bet*x);

c0 = (Bet^2*coshBx + Bet^2*cosBx)/(Bet^2 + Bet^2);c1 = ((Bet^2/(Bet^2*Bet + Bet^3))*sinhBx + (Bet^2/(Bet^3 + Bet*Bet^2))*sinBx)/x;c2 = (1/(x^2*(Bet^2 + Bet^2)))*(coshBx - cosBx);c3 = ((1/(Bet^2*Bet +Bet^3))*sinhBx - (1/(Bet^3 + Bet*Bet^2))*sinBx) /x^3;

c0 = vpa(c0,5);c1 = vpa(c1,5);c2 = vpa(c2,5);c3 = vpa(c3,5);

%% Elements of FTM MatrixF33 = c0;F34 = c1*x;F35 = (c2*x^2)/EI;F36 = -(c3*x^3)/EI;

F43 = (c3*x^3*rhoA*w^2)/EI;F44 = c0 ;F45 = (c1*x)/EI ;F46 = -(c2*x^2)/EI;

F53 = c2*x^2*rhoA*w^2;

F54 = c3*rhoA*x^3*w^2;F55 = c0;F56 = - c1*x;

F63 = -c1*x*rhoA*w^2;F64 = -c2*x^2*rhoA*w^2;F65 = -(c3*x^3*rhoA*w^2)/EI;F66 = c0;

FTM = [F33, F34, F35, F36;F43, F44, F45, F46;F53, F54, F55, F56;F63, F64, F65, F66];

% keyboard FTM = vpa(FTM,10);end

function [Char_Matrix_w,BC_sym,root_cols] = TMM_characteristic_equation_class(BC,GTM_w)% Finds the symbolic characteristic equation for TMM formulation %----------------------------------INPUT-----------------------------------% BC - string, geometric BC on either side ('clamped','pinned','free')% example: 'clamped-clamped' OR 'clamped-free'% GTM_w - global transfer matrix of system (usually a 4x4 matrix)% symbolic in 'w'%---------------------------------OUTPUT-----------------------------------% Char_Matrix_w - characteristic matrix symbolic in 'w'% BC_sym - [4x2] matrix of symbolic B/C for root (column 1) and tip% (column 2)% Sequence is: [ PHIx ; %mode shape% d(PHIx)/dx; %slope% M ; %moment% V ] %shear force% root_cols - Non-zero states at the root%--------------------------------------------------------------------------

%% Actual evaluations start from here

idx = strfind(BC,'-');

BC_tip = BC(idx + 1:end);BC_root = BC(1 : idx-1);BC_vec{1} = BC_tip;BC_vec{2} = BC_root;

syms Phi PHI_x M V

BC_sym = sym(zeros(4,2));states_sym = [Phi;PHI_x;M;V];for i = 1:2 BCi = BC_vec{i}; if strcmp(BCi,'clamped') == 1

states = [0;0;1;1]; elseif strcmp(BCi,'free') == 1

states = [1;1;0;0]; elseif strcmp(BCi,'pinned') == 1

states = [0;1;0;1]; else

error('Either BC is pinned/clamped/free. Pls input only these') end BC_sym(:,i) = states.*states_sym;endtip_rows = find(BC_sym(:,1) == 0); % Zero states at the tiproot_cols = find(BC_sym(:,2) ~= 0); % Non-zero states at the rootChar_Matrix_w = GTM_w(tip_rows,root_cols);

end

function [w_h, nroots] = TMM_root_finding_class(w,Char_Matrix_w, w_guess)assume(w > 0)

% Char_Matrix_w = Char_Matrix_check(Char_Matrix_w,w);Det_F = Char_Matrix_w(1,1)*Char_Matrix_w(2,2) - Char_Matrix_w(1,2)*Char_Matrix_w(2,1);Det_F = vpa(Det_F,5);disp('Characteristic determinant evaluated')w_h = [];

[w_h_temp,fval] = fzero(@(w0) double(subs(Det_F,w,w0)),w_guess);

w_h_temp = double(abs(w_h_temp));w_h = uniquetol(sort([w_h,w_h_temp]),1e-3);w_h = nonzeros(w_h);w_h = transpose(nonzeros(w_h));

if isempty(w_h) == 1 error('No frequencies found within search range !')enddisp('Finished solving Eigenvalue problem.') nroots = numel(w_h);

end

function [PHI,PHIs,PHI_symbolic] = mode_shape_class(w_h,w,FTM_x_w,x,l_s,BC,r)%% Mode shape % Finding the maximum value of a plotted mode shape and then normalizing% with the max valuesyms x[PHI_shape_temp] = get_sym_bending_modeshape(w_h,w,FTM_x_w,x,l_s,BC);%FTM_w,(w_a(i),w,FTM_x_w,x,FTM_w,l_s);PHI_shape = transpose(simplify(expand(PHI_shape_temp)));PHI_shape = vpa(PHI_shape,8);

disp('Finished evaluating symbolic mode shape.')

%% Mode shape normalizationPHI = [];[~,~,n] = size(FTM_x_w); % number of TMM beam segmentsPHI_value = [];for i = 1:n PHI_value_s = double(subs(PHI_shape(i,:), x, r{i})); %segmental mode shape PHI_value = vertcat(PHI_value, PHI_value_s); PHI = vertcat(PHI, PHI_value_s); PHIs{i} = PHI_value_s;end

max_mode_value = max(abs(PHI_value));PHI_symbolic = PHI_shape/max_mode_value;PHI = PHI/max_mode_value;

for i = 1:n PHIs{i} = PHIs{i}./max_mode_value;end

disp('Finished normalizing symbolic mode shape.')

end

function PHI_shape = get_sym_bending_modeshape(w_eval,w,FTM_x_w,x,l_s,BC)%% EVALUATES THE SYMBOLIC MODE SHAPE%This function plots the mode shape corresponding to a natural frequency%----------------------------------INPUT-----------------------------------% w_eval - scalar, current natural freq. (rad/s) for which mode shape is reqd.% w - scalar, symbolic natural freq.% FTM_x_w - Field transfer matrix (symbolic in 'x' and 'w')% x - symbolic beam local coordinate along length% l_s - [1xns] length of each segment% BC - string, geometric BC on either side ('clamped','pinned','free')% example: 'clamped-clamped' OR 'clamped-free'% FTM_size - whether to perform TMM in 4x4 or more general 6x6 format%---------------------------------OUTPUT-----------------------------------% PHI_shape - [1xns] segmental mode shape corresponding to 'w_eval' natural freq.%--------------------------------------------------------------------------% keyboard[nrow, ncol, n] = size(FTM_x_w);FTM_w = sym(NaN(nrow,ncol,n));% [N,~] = size(FTM_x_w);mul = 1;for j = 1:n FTM_w(:,:,j) = subs(FTM_x_w(:,:,j),x,l_s(j)); termj = subs(FTM_w(:,:,j),w,w_eval);%( FTM_w,freq_w,w,ns,BC) mul = termj*mul;endFTM = subs(FTM_w,w,w_eval); %3D matrix 'FTM' is just numbersGTM = double(mul); % GTM = vpa(GTM,5);

[U, ~,root_cols] = TMM_characteristic_equation_class(BC,GTM);PHI_shape = sym(NaN(1,n));

if U(1,2) ~= 0 k = - U(1,1)/U(1,2);elseif U(2,2) ~= 0 k = - U(2,1)/U(2,2);else k = 0;endk = double(k);

%% Current segment is just the beam and all previous segments are beam + point mass FTM's% keyboardfor seg = 1:n GTM_seg_x = eye(nrow); %4x4 matrix if seg > 1

for i = 1: seg - 1 %segmental sweep before current segmentGTM_seg_x = FTM(:,:,i)*GTM_seg_x; %at this point this matrix 'GTM_seg_x' is just numbers

endGTM_seg_x = subs(FTM_x_w(:,:,seg),w,w_eval)*GTM_seg_x; %at this point this matrix 'GTM_seg_x' is just a function of 'x'

elseGTM_seg_x = subs(FTM_x_w(:,:,seg),w,w_eval); %for a structure with just 1 segmentGTM_seg_x = vpa(GTM_seg_x); %4x4 matrix OR 6x6 matrix

end

PHI_shape(seg) = GTM_seg_x(1,root_cols)*[1; k];

end

end

function plot_modeS = mode_shape_plot_panel_class(figno,PHI,PHIs,n,mode_type,x_vec,BC,omega)

legend_type = 'n_h';mode_symbol = '\phi_w(x)';

plot_modeS = figure(figno);set(plot_modeS,'defaulttextinterpreter','latex')

for i = 1:n

plot(x_vec{i}, PHIs{i},'LineWidth',2) hold onend

% plot(x_vec, PHI,'LineWidth',2);% legend_stringh = ['f_h',' = ',num2str(0.5*omega/pi,5),' [Hz]'] ;

grid on% legend(legend_stringh);

title(['[',BC,']',' $f_n',' = ',num2str(0.5*omega/pi,5),'$ [Hz]'])xlabel('$Structure\,\,Length(m)$')ylabel(['$',char(mode_type),'\,\,mode\,\,shape\,\,',mode_symbol,'$']) %'% set(legend,'Location','Best','LineWidth',1,'FontSize',9)

end

E_s = 200e9; % Pa, substrate elastic modulusrho_s = 7800; % kg/m^3, substrate densityE_p1 = 120e9; % Pa, patch 1 elastic modulusrho_p1 = 8600; % kg/m^3, patch 1 density

% Beam Substrate Geometryw = 0.02; % mts = 0.001; % mL = 0.3; % m

% Patch GeometryLp1 = 0.04; % m, patch 1 lengthtp1 = 0.0015; % m, patch 1 thicknessx1p1 = 0.07; % m, patch 1 start positionx2p1 = x1p1+Lp1; % m, patch 1 end position

% Specify Applied Forcef = 0;

% Define mode shape coefficients for cantilevered beamlam_L = [3.92660231 7.06858275 10.21017612 13.35176878 16.49336143];sig = [1.0008 1.0000 1.0000 1.0000 1.0000];if N_modes > 5 for i = 5:N_modes

lam_L = [lam_L ((4*i+1)*pi/4)];sig = [sig 1];

endend

Rayleigh-Ritz code

function Rayleigh_Ritz_20ticclcformat short

N_modes = 20; % Different cases N = [1 2 3 4 5 10 15 20 30]

% Material Properties

lam = lam_L/L;

% Compute System Stiffness, Mass, and Force Input MatricesKs = zeros(N_modes,N_modes); Kp1 = zeros(N_modes,N_modes); % Preallocate matrixes to speed up matlab

Ms = zeros(N_modes,N_modes); Mp1 = zeros(N_modes,N_modes);

for i = 1:N_modes for j = 1:N_modes

Ks(i,j) = E_s*triplequad(@(x,y,z)K_integrnd(x,y,z,i,j,sig,lam,L),0,L,-w/2,w/2,-ts/2,ts/2);Kp1(i,j) = E_p1*triplequad(@(x,y,z)K_integrnd(x,y,z,i,j,sig,lam,L),x1p1,x2p1,-w/2,w/2,ts/2,ts/2+tp1);

Ms(i,j) = rho_s*triplequad(@(x,y,z)M_integrnd(x,y,z,i,j,sig,lam,L),0,L,-w/2,w/2,-ts/2,ts/2);Mp1(i,j) = rho_p1*triplequad(@(x,y,z)M_integrnd(x,y,z,i,j,sig,lam,L),x1p1,x2p1,-w/2,w/2,ts/2,ts/2+tp1);

end endK = Ks+Kp1; % system stiffness matrixM = Ms+Mp1; % system mass matrix

% compute natural frequenciesK_til2 = M\K;wn = sort(eig(K_til2).^(1/2));fn = wn/(2*pi);f1 = fn(1)f2 = fn(2)f3 = fn(3)tocend

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Integrand Functions %%%%%%%%%%%%%%%function int_K = K_integrnd(x,y,z,i,j,sig,lam,L)xi = x;d2phidx_d2phidx = (lam(i)^2*(cosh(lam(i)*xi)+cos(lam(i)*xi)-sig(i)*(sinh(lam(i)*xi)+sin(lam(i)*xi)))).*(lam(j)^2*(cosh(lam(j)*xi)+cos(lam(j)*xi)-sig(j)*(sinh(lam(j)*xi)+sin(lam(j)*xi))));int_K = z.^2.*d2phidx_d2phidx+0.*y; % Integrand is not a function of y, but matlab needs y to be includedend

function int_M = M_integrnd(x,y,z,i,j,sig,lam,L)

xi = x;phi_phi = (cosh(lam(i)*xi)-cos(lam(i)*xi)-sig(i)*(sinh(lam(i)*xi)-sin(lam(i)*xi))).*(cosh(lam(j)*xi)-cos(lam(j)*xi)-sig(j)*(sinh(lam(j)*xi)-sin(lam(j)*xi))); % modes for cantilevered beamint_M = phi_phi+0.*y+0.*z; % Integrand is not a function of y or z, but matlab needs them to be includedend